SILRES^® MK POWDER

1. Glass Fibre Laminates

Please note: manufacturing prepregs and laminates is a complex process that needs a thorough control of various process parameters, such as temperature, catalyst quantity, nature of catalyst, solvent type, machine specific conditions, etc. Therefore, the following indications are inteded as a guide only, and we recommend running preliminary tests to optimize the conditions of the particular process.

Since fully cured SILRES® MK POWDER provides water repellency, excellent electrical insulation properties, outstanding heat resistance and long term stability against weathering, moisture and UV light, this silicone resin is often used as a binder for laminates made of fibrous fillers, cloths, woven or non-woven reinforcing materials. The following process illustrates a typical production sequence for the manufacture of glass cloth laminates.

SILRES® MK POWDER is dissolved in toluene, or any other suitable organic solvent, to form a resin solution with a solids content of about 10-20 % by weight. The curing catalyst is added right prior processing; for the purpose of storage-stable prepregs WACKER® Catalyst F,100%, WACKER® Catalyst K83, zinc acetylacetonate or related metal salts of acetylacetonate and of long-chained carboxylic acids proved most useful as catalysts.

The glass cloth is then impregnated by the catalyzed silicone resin solution either by spraying, or by running the fabric through an impregnation bath. The solvent subsequently is evaporated in a drying oven at moderate temperature (130 to 150 °C). Please note: the drying temperature must not be too high in order to avoid premature curing of the B-stage resin and of the prepreg web, respectively. Also a very long drying cycle can impair the storage stability of the prepregs made.

Depending on the target shape of the laminate product the prepreg web either is rolled-up in layers on a stainless steel mandrel, or it is cut up into smaller pieces which in turn are sandwiched to a layered stack. By applying heat (e.g. 200 °C) and high pressure (for the layered stack assembly: >10 bar) the B-stage resin converts into the C-stage to form a durable and flexurally very rigid composite with very high heat resistance.

Please note: inserting a PTFE coated glass fabric between the pressing plates and the layered prepreg stack can help to prevent the composite from sticking to the plates. Besides it has proven useful to first cool the composite down to 80 °C (or lower) before the laminating press is opened, as this minimizes the delamination risk during the decompression phase.

2. Mica Laminates

Please note: manufacturing prepregs and laminates is a complex process that needs a thorough control of various process parameters, such as temperature, catalyst quantity, nature of catalyst, solvent type, machine spedific conditions, etc. Therefore, the following indications are inteded as a guide only, and we recommend running preliminary tests to optimize the conditions for the particular process.

The process of making mica laminates is similar to the production of glass fibre laminates. Important: the quality of the final product strongly depends on the type of mica, its particle size distribution and the uniformity of the mica paper used.

Similar to the process described for glass fibre laminates, the binder-free mica paper is first impregntated by a catalyzed solution of SILRES® MK POWDER in toluene or any other suitable organic solvent (solids content of about 10-20 % by weight). After a short residence time the solvent is evaporated in a drying oven at moderate temperature (130 to 150 °C) to yield a physically dry, prepreg web that can be rolled-up in layers for storage.

For producing mica laminates, the prepreg web is cut up into smaller pieces, sandwiched to a layered stack of desired thickness and pressed for approximately 1 hour at 200 °C under high pressure (>10 bar).

Please note: inserting a PTFE coated glass fabric between the pressing plates and the layered prepreg stack can help to prevent the composite from sticking to the plates. Besides it has proven useful to first cool the composite down to 80 °C (or lower) before the laminating press is opened, as this minimizes the delamination risk during the decompression phase. Pressureless post-curing at 250 °C can further improve the lmainate's properties in terms of hardness and cohesive strength.

3. Hydrophobation of Magnesium Oxide Filler for Tubular Heating Elements

Fully cured SILRES® MK POWDER provides high water repellency, excellent electrical insulation properties and outstanding heat resistance. It is therefore often used to impregnate magnesium oxide powder which in tubular heating elements acts as heat transfer medium and as dielectric packing material for the heating wire.

To improve the magnesium oxide's moisture repellency the milled powder is mixed with up to 2 weight % of SILRES® MK POWDER; a curing agent is not necessary. The powder mixture is then filled into the heater tube with inserted heating wire, followed by the compacting and shaping steps. During the first heating the silicone resin powder liquifies and impregnates the filler particles to make them hydrohobic. By this water-repellent packing the heating wire is safely protected against moisture and corrosion for the entire life cycle of the tubular heating element.

4. Binder for Resistor Packings and for Resistor/Capacitor Coatings

As SILRES® MK POWDER can be highly filled with mineral fillers, it often is used as binder for resistor packing compounds or in coatings for resistors and capacitors. The respective silicone resin based blends are highly fire-retardant, and they safely protect the electrical parts from moisture or any other environmental impact. Besides the thermal management is improved, which extends the lifecycle of the particular electrical element - even when continuously run at high temperature levels.

A - Production of packing compounds for cement resistors:

In a mixing kneader solid SILRES® MK POWDER is blended with finely ground inorganic fillers and pigments to give a dry powder mixture with a silicone resin content of about 10 to 20 % by weight; if applicable, glass fibres and additive powders can be used additionally. Then solid catalyst powder, e.g. WACKER® Catalyst F 100% or metal salts of acetylacetonate or of long-chained carboxylic acids, is admixed in a quantity of 1.5 to 3 wt.%, based on the amount of solid silicone resin used. Please note: due to the low softening point of SILRES® MK POWDER it is recommended to cool the mixing device in order to prevent premature agglomeration of the powder blend. 

By elevating the mixing temperature above the softening point of the silicone resin, the dry powder mix can be compacted and pelletized for storage. When heated above 100 °C the pellets can be used to pack the resistor body by transfer-molding. Baking the green body at high temperature (150 °C, 90 min.) finally gives a hard, durable cement composite that safely protects the resistor body from external impacts.

B - Production of resistor and capacitor coatings:

SILRES® MK POWDER is dissolved in toluene, xylene or any other suitable organic solvent, to form a resin solution with a solids content of about 25 to 35 % by weight. Then the finely ground fillers and pigments, which in total account for up to 75 % of the final coating formulation, are added and thoroughly mixed in a kneader. Finally, a curing catalyst (e.g. WACKER® Catalyst F 100%, WACKER® Catalyst K 83 or GENIOSIL® GF 91) is admixed in a quantity of some 1.5 to 3 wt.%, based on the amount of solid silicone resin used. If necessary, more solvent can be added to adjust the consistency of the coating slurry to the particular needs.

For coating the respective electrical part is dipped into the slurry; alternatively, resistors can be coated by rolling them over the surface of the slurry. The solvent subsequently is evaporated at moderate temperature, e.g. in a drying oven. Finally the green body is cured at high temperature (150 °C, 90 min.) to give a hard, durable coating of 0.5 to 1 mm thickness. Note: to increase the coating thickness multiple dip-coating is possible.